In assembly lines for automobiles, home electric appliances, and the like, spot welding methods, which have high working efficiency among resistance welding methods, are widely employed. In a mass production line, continuous spot welding is carried out. An electrode for continuous spot welding is repeatedly exposed to high heat and high charge during use, so deformation thereof is likely to occur; therefore, excellent deformation resistance is required for an electrode raw material. The electrode raw material also needs to be excellent in electric conductivity, thermal conductivity, strength, and abrasion resistance, which are the inherently required properties of electrodes for spot welding. Accordingly, a Cu alloy such as a Cu—Cr alloy or a Cu—Cr—Zr alloy or a Cu material in which a hard substance such as Al2O3 is dispersed is used as the electrode raw material. Of those, Cu—Cr alloys are widely used from comprehensive viewpoints of thermal conductivity, strength, cost, and the like.
However, as a raw material for automobiles, home electric appliances, and the like, plated steel sheet plated with Zn, Zn alloy, or the like has been widely used in order to enhance durability of the raw material. The plated steel sheet is spot welded by being supplied with heavier current compared with current in spot welding of cold-drawn steel sheet; therefore, the distal end part of the electrode is exposed to further severe conditions.
At the distal end of the electrode during welding, an alloying reaction takes place between plated layer constituents such as Zn and Al, Fe which is a matrix constituent of plated steel sheet, and Cu which is a main constituent of electrodes, whereby intermetallic compounds such as Cu—Zn, Cu—Zn—Al—Fe, and the like are easily formed. The generated intermetallic compounds are so brittle that the intermetallic compounds are peeled off by the pressing at the time of welding. The phenomenon brings about an increase in diameter of the distal end of the electrode, which is a cause of lowered current density.
As a result, in the case of welding plated steel sheet, the lifetime of the electrode becomes shorter compared with the case where cold-drawn steel sheet such as ordinary steel or stainless steel is spot welded. The shorter lifetime of the electrode has a great adverse effect on the workability of spot welding, wherein the number of used electrodes is increasing.
Accordingly, the inventors of the present invention previously proposed an electrode for spot welding having a double structure including a core material embedded in the central part of the distal end of the electrode, aiming for a longer lifetime of the electrode.
An electrode for spot welding according to Patent Document 1 includes Cu or a Cu alloy as an electrode body and a core material made of W, Mo, a W-based alloy, or a Mo-based alloy embedded in an abutting surface, which is a surface of the electrode that abuts against a material to be welded, so the area ratio of the core material to the abutting surface is adjusted to the range of 0.7 to 3.0. In the core material, at least one kind of fine particles selected from an oxide, a nitride, a carbide, and a boride of a 2A group element, a 4A group element, a 5A group element, a 6A group element, or a rare earth element may be dispersed at a ratio of 0.5 to 10 volume %.
An electrode for spot welding according to Patent Document 2 is a double-structured electrode developed for subjecting a Mg-containing Zn-based alloy-plated steel sheet to spot welding, and the electrode includes, for a core material thereof, W, Mo, a W-based alloy, or a Mo-based alloy in which one kind or two or more kinds of fine particles selected from oxides of Be, Mg, Ca, Sr, Ti, Zr, Y, and Ce are dispersed at a ratio of 0.5 to 10 volume %.
An electrode for spot welding according to Patent Document 3 also includes, for a core material which is one of the constituents of a double structure, a W-based alloy in which fine particles are dispersed at a ratio of 0.5 to 10 volume % in total, the fine particles being one kind or two or more kinds of compounds selected from an oxide, a nitride, a carbide, and a boride of a 2A group element, a 4A group element, a 5A group element, a 6A group element, and a rare earth element, and the fine particles each having a melting point of 2,400° C. or higher and an average particle diameter of 2 μm or less.
Each of the electrodes maintains relatively high strength (high hardness) even under high temperatures, and have a double structure including the core material made of W, Mo, the W-based alloy, or the Mo-based alloy, which resists alloying reactions with plated metal, embedded in the electrode body made of Cu or the Cu alloy. The core material made of W, Mo, the W-based alloy, or the Mo-based alloy also easily ensures a current-carrying path of a certain size of area by being embedded in the electrode body; therefore, the lowering of welding current density can be suppressed, and an electrode having a longer lifetime can be aimed at.
W and Mo, which are originally hard materials, have been considered to have a defect in that, during spot welding, cracks are generated by impact at the time of pressing the electrode so that the W and Mo are easily broken. However, through a pinning action against crack development caused by the added fine particles, the generation and propagation of the cracks can be suppressed. By adding the fine particles, a large part of the core material does not break, enlargement of the current-carrying path is suppressed, and an approximately constant nugget diameter can be obtained. Owing to those effects, the lifetime of the electrode is improved to a large degree compared to that of a conventional electrode made of a Cu alloy.    Patent Document 1: Japanese Patent Laid-Open No. 2006-15349    Patent Document 2: Japanese Patent Laid-Open No. 2006-95549    Patent Document 3: Japanese Patent Laid-Open No. 2006-102775